The physiologic role of vitamin D will be investigated by measuring the rate of protein and DNA synthesis in a variety of tissues considered not to be involved in calcium and phosphorus homeostasis. These studies will be performed in tissue culture to eliminate the influence of systemic changes induced by the vitamin and the further metabolism of the specific role of the serum and cytoplasmic vitamin D binding protein in mediating the action of the sterols. The influence of vitamin D on phosphate transport will be investigated primarily in intestinal explants and isolated intestinal mucosal cells maintained in culture. The influence of vitamin D on muscle phosphate transport and phosphate homeostasis will be studied in vitro and in vivo. These studies are designed to demonstrate the role of vitamin D in the maintenance of cell function and growth.